1. Field of the Invention
The present invention relates generally to a recording circuit suitable for a video tape recorder (VTR), video disc apparatus or the like, and, more particularly, is directed to a recording circuit which can remove an undesired frequency component that causes an undesirable beat interference.
2. Description of the Prior Art
When a color video signal is recorded on a magnetic tape by a video tape recorder (VTR) which, for example, employs a carrier chrominance signal low frequency converting system, the color video signal is separated into a luminance signal and a chrominance signal, the luminance signal is frequency-modulated to provide an FM luminance signal, and the chrominance signal, originally having a carrier frequency of 3.58 MHz in the case of the NTSC system, is low frequency converted to a low frequency converted carrier chrominance signal having a carrier frequency of, for example, 668 kHz. Then, the FM luminance signal and the low frequency converted carrier chrominance signal are mixed or combined and the resulting combined video signal is recorded on the magnetic tape.
When a video signal thus recorded on the magnetic tape is played back or reproduced a combined video signal reproduced from the magnetic tape by a magnetic head is separated into the FM luminance signal and the low frequency converted carrier chrominance signal. The FM luminance signal is demodulated to provide the luminance signal, and the low frequency converted carrier chrominance signal is frequency converted back to the original carrier frequency of 3.58 MHz. Then, the demodulated luminance signal and the reconverted carrier chrominance signal are mixed to provide a reproduced color video signal.
In order to obtain the FM luminance signal having a center frequency f.sub.r from the combined video signal reproduced by the head, it is proposed to remove the low frequency converted carrier chrominance signal having a carrier frequency f.sub.c by means of a trap circuit, or to pass only the FM luminance signal through a high-pass filter. Since the low frequency converted carrier chrominance signal has no frequency deviation, and the frequency band thereof is narrow, the low frequency converted carrier chrominance signal can be readily removed by the trap circuit.
However, a problem arises when using a trap circuit, as described above. More specifically, a cross color may occur when a recorded video signal enters the carrier chrominance signal to give color to an uncolored portion of the picture. In such case, it is difficult to separate the carrier chrominance signal from the recorded color video signal.
In order to avoid the above-mentioned problem, it has been proposed to connect a trap circuit, having a trapping frequency corresponding to that of the low frequency converted carrier chrominance signal, to the output of the frequency modulator for the luminance signal before the resulting FM luminance signal and the low frequency converted carrier chrominance signal are mixed and then recorded on a magnetic tape.
FIG. 1 shows an example of a circuit which embodies the above-mentioned arrangement according to the prior art, and in which a luminance signal is applied to an input terminal 1. The luminance signal applied to the input terminal 1 is FM-modulated by an FM modulator 2 and fed to one input of an adder 3, in which it is added or combined with a low frequency converted carrier chrominance signal supplied to the other input of the adder 3 from a terminal 4. The resulting combined or mixed signal from the adder 3 is recorded on a magnetic tape 6 by a magnetic head 5. Further, in the prior art arrangement of FIG. 1, a trap circuit 7 is interposed between the FM modulator 2 and the respective input of the adder 3. The frequency of the trap circuit 7 is selected to be substantially the same as that of the signal to be mixed with the FM luminance signal, that is, the low frequency converted carrier chrominance signal.
FIG. 2 schematically shows a frequency spectrum of a standard FM signal derived from the FM modulator 2, and from which it is apparent that the odd-numbered side band components of the lower side band (LSB) and the upper side band (USB) are opposite in phase, whereas the even-numbered side band components of the lower and upper side bands are the same in phase as each other and as the carrier. Since the FM luminance signal and low frequency converted carrier chrominance signal are mixed by the adder 3 and then recorded by the circuit shown in FIG. 1, a frequency spectrum as shown in FIG. 3A has to be considered as possible for the output from the FM modulator 2. In other words, an odd-numbered side band component having a frequency equivalent to the frequency f.sub.c of the low frequency converted carrier chrominance signal may occur in the LSB, and may interfere with the low frequency converted carrier chrominance signal when mixed with the latter in the adder 3, thereby causing a beat interference. In an effort to avoid such interference, the prior art provides the trap circuit 7 whose trap frequency corresponds to the frequency f.sub.c of the odd numbered side band components which is the same as the frequency f.sub.c of the low frequency converted carrier chrominance signal with which the FM luminance signal is to be mixed. The odd-numbered side band component of the frequency f.sub.c in the LSB is removed by the trap circuit 7, as shown in FIG. 3B. However, if the recording/reproducing system that follows the adder 3, such as, the magnetic head 5, the magnetic tape 6 and the like, has a limiter characteristic, the USB side of the FM luminance signal is folded in the course of the recording and reproducing operations, whereby the odd-numbered side band component which had been trapped in the LSB is restored therein, as shown in FIG. 3C. The frequency f.sub.c of the restored odd-numbered side band component is close to that of the low frequency converted carrier chrominance signal, so that this odd-numbered side band component and the low frequency converted carrier chrominance signal cause a beat interference to occur which deteriorates the quality of the reproduced picture.